1. Field of the Invention
The present invention is concerned with the allocation of spectral bands to the upstream and downstream directions in twisted pair modem communications. Specifically, the invention aims at a spectral management scheme that minimizes interference among transmissions in different pairs and can accommodate mixtures of symmetric and asymmetric services in the same binder group of twisted pairs. The invention further relates to a dynamic and distributed spectral management paradigm that can be implemented through a set of simple rules.
2. Prior Art
Communication systems that interconnect users over twisted pair wireline connections are being developed to support broadband data communication. Recent developments in broadband communication protocols allow broadband data to be overlaid on these existing twisted pair connections that also carry narrowband voice or integrated service digital network (ISDN) traffic. Specifically, the interconnection of modems allows broadband data to be communicated on unused frequency channels of the existing twisted pair lines. These unused or spare frequency channels are isolated from the conventionally encoded voice signals by a suitable filter.
Depending upon the complexity of the coding scheme, overlaid broadband systems can support data rates in excess of two Megabits per second (Mbps), although this rate is dependent upon the physical parameters of the connection, e.g. the overall length of the twisted pair and its composition and configuration. Asymmetric Digital Subscriber Line (ADSL) and High-speed Digital Subscriber Line (HDSL) protocols, for example, can support data rates of 2 Mbps over distances of approximately three kilometers, while more complex schemes (such as VDSL) can support data rates of 8 Mbps and above over distances of, typically, less than two kilometers.
Some xDSL systems employ a time division duplex transmission scheme in which a communication resource (such as a dedicated channel within frequency limits) has a time-split use for up-link and down-link transmissions between line termination equipment and the customer equipment. More specifically, the up-link and down-links may have different traffic capacities. For example, in an Internet-type environment, it is usually beneficial to have a higher down-link capacity since information download is the dominant data flow, whereas general business traffic generally requires equal traffic capabilities in both directions.
In relation to bundles of twisted pair wireline communication resources, it is also important to consider the potentially undesirable effects associated with cross-talk interference. Specifically, with bi-directional communication, the relative location of the lines, for example, between twisted copper-pair causes cross-talk interference to be induced into proximately located wireline communication resources (principally by the mechanisms of capacitive and inductive coupling and by radiation arising from the imperfect nature and performance of the cabling). Moreover, where symmetrical and asymmetrical services are simultaneously required on pairs in the same bundle, cross-talk becomes a significant problem.
One type of interference in these types of systems is referred to as near-end cross-talk or NEXT. NEXT occurs when electromagnetic interference is induced into a twisted pair wireline resource that is communicating information in an opposing direction, e.g. downlink (or downstream) information appears as noise in an uplink (or upstream) path. NEXT is undesirable because near-end generated interference is at a level that can potentially swamp data signals received from a remote terminal, which data signals have previously been subjected to attenuation through the transmission path. NEXT is generally produced by local end amplifiers. Furthermore, NEXT increases significantly at the higher frequency components and so is even more undesirable in high frequency data-over-voice wireline systems, such as VDSL. To avoid the harmful effects of NEXT in a TDD system, an ensemble of collated communication resources must have synchronized and aligned transmissions. However, in a mixed symmetrical-asymmetrical system, NEXT often occurs where the two opposing schemes have either different frequency allocations (in frequency division duplex, FDD) or different time slot allocations (in TDD).
Another kind of interference in these types of communications systems is referred to as Far End Cross-Talk (FEXT). This form of cross-talk affects non-addressed ports of a remote terminal. In other words, FEXT occurs when electromagnetic interference (i.e. noise) is induced into a wireline resource that is communicating information in a similar direction, e.g. upstream (or up-link) information appears as noise in another upstream wireline resource to an extent that performance on a given pair is limited. The effects of FEXT are correspondingly reduced by the attenuation path of the wireline resource. However, when multiple separate modem links exist, crosstalk between the numerous signals at an exchange point generates noise that limits the data-rate performance of both a given pair and the entire wireline system, in general.
FIG. 1 shows the interference issues that arise in prior art cables when multiple twisted copper pairs are bundled together in binder groups. The cable 10 contains line 1 and line 2 that are transmitting data as described above. In this example only interference between two pairs is depicted. Near end crosstalk II (NEXT) is induced on both ends of the cable 10, while far end crosstalk 12 (FEXT) resulting from the far end transmitters, also couples into both lines 1 and 2, and is in both directions of transmission. In many cases NEXT is more detrimental than FEXT especially in medium and long length loops. Spectral management techniques are frequently evoked to mitigate the NEXT interference problem.
FIG. 2 demonstrates a prior art frequency division multiplexing approach (FDM), where the available bandwidth 20 on each line is divided for upstream transmission 21 and 23, and downstream transmission 22 and 24. In this scheme the upstream and downstream transmission bands are separated in frequency. In this type of FDM the induced NEXT is out of band in each direction and hence is of little or no concern. FEXT is still present in both directions and is shown by the shaded areas 25 and 26. This scheme works well if all the pairs in the binder deliver the same service and hence have the same FDM band plan. For example, this spectral band plan and this separation works well when all pairs in the binder deliver ADSL service to residences.
Unfortunately, in many instances there is a need for mixing asymmetric and symmetric services in the same binder (e.g., residential and business services in the same neighborhood). FIG. 3 shows cables 31 and 32 that transmit symmetrically along line 1 and asymmetrically on line 2. The NEXT interference is depicted by arrows 33 and 35. The FEXT interference is indicated by arrows 34 and 36. In the downstream direction in 31 the asymmetric service transmitter uses more bandwidth and induces NEXT into the symmetric service receiver. Conversely, on the customer side the symmetric service transmitter uses more bandwidth upstream and induces NEXT into the asymmetric receiver.
FIG. 4 shows the resulting interference problems in the prior art when two pairs use different symmetry ratios and hence band plans that are not perfectly aligned. The available bandwidth used for transmission is shown in 40. Line 1 has upstream bandwidth 41 and downstream bandwidth 42. Line 2 has upstream bandwidth 43 and downstream bandwidth 44. As shown by shading 45, there now exists overlap between the upstream 41 and downstream 44 bands of the different pairs and NEXT interference 45 is not completely out of band.
These problems are accentuated when the different interacting pairs have different loop lengths. More intelligent spectral management techniques are needed so that modems minimize their spectral pollution whenever possible. The most straightforward way to contain the interference problem is to define static spectral management rules and fixed spectral masks that every modem should follow. The ANS1 T1.417 Spectral Management Standard, Issue 2, 2003, is one example of this approach. While the problem is not eliminated, some safeguards are put in place so that interference does not get out of hand.
More advanced approaches are based on the modems dynamically adjusting their spectra based on noise and interference conditions. These dynamic spectral management schemes (DSM) impose rules that instruct the modems to not transmit excessive amounts of energy when not necessary as detailed in “Dynamic Spectral Management (DSM) Technical Report, Committee T1E1.4, 2003”. The simplest form of DSM requires the modems to voluntarily back off their transmit signal power when operating with excess SNR (also called excess SNR margin). This “good citizen” behavior results in less spectral pollution overall and benefits everybody.
Recently, a more involved DSM approach has been proposed called “iterative waterfilling”. It is also based on the idea of each modem voluntarily performing power back-off when possible but each time this happens, the modem re-distributes its available power across the frequency band using the well known communications theory principle of waterfilling as in U.S. Patent Application 20030086514, May 8, 2003. In simple words, this process puts more energy where the SNR is higher and less energy where the SNR is lower. This method holds some promise for improved crosstalk protection. It was recently shown however in Jan Verlindens paper “Target PSD Obtained With Iterative Waterfiling is Almost Flat”, T1E1.4/2003 contribution no. 295, that given the way iterative waterfilling is implemented in DSL modems (with DMT modulation) the results are very similar to those of a simple power back-off.
All these DSM methods are well suited to situations that are FEXT dominated (e.g., VDSL systems on short loops). For systems on longer loops however, FEXT is only a secondary concern, compared to the effects of NEXT. Power back-off methods do not specifically address or correct the NEXT interference issues as shown in FIG. 4.